This invention relates to quadraphonic sound systems, and more particularly to a system for producing from surround-sound sources two composite signals which when decoded by an appropriate four-channel decoder reproduce the directional characteristics of the original sound sources.
In copending patent application Ser. No. 685,065, filed May 10, 1976, now U.S. Pat. No. 4,072,821, the present applicant has described a microphone system for producing signals for quadraphonic reproduction which includes four coaxial microphone transducers which typically define limacon patterns of revolution corresponding to the equation, .rho.(.theta.)=0.3+0.7cos.theta. where .rho. is the fraction of the maximum sensitivity of the sensor as a function of angular deviation .theta. from the positive direction of the axis of revolution. As described in connection with FIGS. 14 and 15 of the aforementioned copending application, a composite of which is presented in FIG. 1 of the accompanying drawings, the axes of maximum sensitivity of the four sensors typically are coplanar and are arranged azimuthally around a common axis such that one of the units, designated L1, is aimed at -65.degree., a second unit designated R1 is aimed at +65.degree., a third unit, designated L2, is aimed at -165.degree., (a third unit, designated L2, is aimed at -165.degree.,) and a fourth unit, designated R2, is aimed at +165.degree..
The output from each of the two "front" sensors L1 and R1 is passed through a respective all-pass phase-shift network having a phase-shift angle that varies as a function .psi. of frequency. Similarly, the output signal from each of the two "back" sensors is passed through a respective all-pass network having a phase shift angle that varies as a (.psi.-90.degree.) function of frequency. A predetermined fraction of the phase-shifted output of sensor R2 is subtracted from the phase-shifted output of sensor L1 to form a "total" or transmitted composite signal designated L.sub.T, and a predetermined fraction of the phase-shifted output of the sensor L2 is subtracted from the phase-shifted output of sensor R1 to form a second composite signal, designated R.sub.T. The composite signals L.sub.T and R.sub.T represent a coded quadraphonic output which, for specific directions of sound arrival in space correspond to the SQ code for the directions left back, left front, center front, right front and right back; for the center back direction the code is the same as for center front, so that the performance of the FIG. 1 system corresponds to that of a "forward-oriented" SQ encoder. The described system is particularly useful for the recording and/or transmitting of a dramatic presentation since it allows the performers to be positioned, and to walk around the microphone array while reproducing their positions from appropriate directions over a wide arc in space. It is shown in the aforementioned application that the respective polar patterns and the respective directions of maximum sensitivity of the limacons, and the relative contributions of the "front" pair, L1 and R1, and the "back pair", L2 and R2, can be adjusted over a relatively wide limit while still achieving the desired encoding performance.
In the system described in the aforementioned application, the four limacon patterns are obtained by using four gradient transducers and one omnidirectional transducer. The gradient transducers typically are arranged coaxially in their positive direction of maximum sensitivity at the aforementioned azimuth angles of .+-.65.degree. and .+-.165.degree., each furnishing approximately 70% of the signal output with sound incident from these directions, with the omnidirectional transducer furnishing the remainder, or about 30%, of the signal output, the latter being added equally to the outputs of the four gradient transducers.